docs: def vssa(self):
          r'''The volume-specific surface area of a particle size distribution.
          Note this uses the diameters provided by the method `Dis`.
          
          .. math::
              \text{VSSA} = \sum_i \text{fraction}_i \frac{SA_i}{V_i}
          Returns
          -------
          VSSA : float
              The volume-specific surface area of the distribution, [m^2/m^3] 
  
          References
          ----------
          .. [1] ISO 9276-2:2014 - Representation of Results of Particle Size 
             Analysis - Part 2: Calculation of Average Particle Sizes/Diameters  
             and Moments from Particle Size Distributions.
          '''
          ds = self.Dis
          Vs = [pi/6*di**3 for di in ds]
          SAs = [pi*di**2 for di in ds]
          SASs = [SA/V for SA, V in zip(SAs, Vs)]
          VSSA = sum([fi*SASi for fi, SASi in zip(self.fractions, SASs)])
          return VSSA